"He literally is a genius, as is evident from his songwriting," she said.

"I always tell him, 'Darling, you know I'm going to have to give your mind to science after you've passed, because we're going to have to understand how all these sparks work'.

"We'll be in bed, and he'll be doing the crossword puzzle. Every night, he tries to finish it in under 10 minutes. When he puts his mind to something, he really gets it done very well. I always ask for his help."

And she also opened up about her changing image - and her ambitions for her future career.

"I love Madonna to death, but she's never going to give me that damn baton," she joked.

"I'll probably turn into more of a Joni Mitchell. As I inch towards my 30s, I think my fourth record will be more of an acoustic guitar album.

"That's where I started when I was first discovered by Glen Ballard and got my first record deal. We'll see - I can't get ahead of myself.

"I'm still doing the work: I'm a good balance of left and right brain, and to be an artist with a long career, you've got to have both. One thing John said to me was, 'It's harder maintaining success than finding it.'

"I've got a few records under my belt, and I still feel like a brand-new artist. People still want that truth to cut through."